The manufacture of certain cellulose pulp products involves subjecting pulps to extraction with a caustic solution, formed from alkali metal salt, such as sodium hydroxide or potassium hydroxide, or other basic compounds such as ammonium hydroxide. This extraction process is preferably conducted at temperatures in the range of about 15° C. to 40° C. and is commonly referred to in the art as cold caustic extraction. It is usually performed by mixing a low consistency cellulose pulp slurry with an alkaline solution and allowing the mixture to soak until the hemicelluloses in the pulp are dissolved and diffuse out of the pulp fibers. The extracted cellulose pulp fiber is then washed to remove the alkaline/caustic solution and the dissolved hemicelluloses. The effluent stream produced by this extraction and washing, referred to herein as a hemicaustic extract, can be further processed by passing it through filtration membranes to increase its hemicellulose concentration. The presence of hemicelluloses in hemicaustic extract makes the cold caustic effluent a possible source of hemicelluloses. The caustic extraction process is described in U.S. Pat. Nos. 6,896,810, 7,812,153 and 7,919,667, all of which are hereby incorporated by reference.
Like cellulose, hemicellulose is a polysaccharide found in wood and other plant materials. It differs from cellulose in many ways. Cellulose is normally fibrous and partially crystalline. Hemicelluloses are non-fibrous and amorphous. Structurally, cellulose is a long chain polymer of anhydroglucose monomer units with a number average degree of polymerization (DP) from 3,000 to 5,000. Hemicelluloses are relatively short chain polymers of predominately mannose and xylose monomers. Cellulose is a homopolysacharide while hemicelluloses are heteropolyhydroxylated polymers. The sugar moiety of both cellulose and hemicelluloses chains bear free hydroxyl groups that are available for chemical reaction. They are capable of undergoing reactions which are typical for primary and secondary alcohols. Heterosacharides can be derivatized to produce materials that can be used as thickeners for foods, coatings, paints, explosive slurries, oil well fluids, cosmetics and other personal care products, and many other products.
Ether derivatives of polyhydroxyl polymers are obtained by chemical reaction of the hydroxyl groups of the monosaccharide moiety with alkylene oxides (ethylene, propylene, butylene or higher oxides) or alkyl chlorides, in the presence of an alkaline catalyst (such as sodium hydroxide). These derivatization reactions may be conducted under heterogeneous conditions in the substantial absence of water or solvent, although the efficiency of the reaction is low without the presence of water. Accordingly, polyhydroxyl polymers are generally derivatized in the presence of a small amount water to provide better reactivity. For instance, carboxymethyl hydroxyalkyl derivatives of polyhydroxyl celluloses can be obtained by chemical reaction of the hydroxyl groups of hemicellulose chains with a mixture of alkylene oxides (ethylene, propylene, butylene or higher oxides) and chloroacetic acid or a salt thereof, such as sodium chloroacetate, in the presence of water and an alkaline catalyst.
Utilization of hemicaustic effluents has been the subject of considerable research. For example, U.S. Pat. No. 7,812,153 describes a process for extracting hemicelluloses from a hemicaustic stream and the use of the hemicaustic so obtained in the manufacture of xylose, which is then hydrogenated to xylitol. U.S. Pat. No. 3,988,198 discloses a method of treating spent hemicaustic effluents so that they can be used for cooking, bleaching or the caustic extraction of pulps. The methods previously investigated included both physical and chemical procedures, but none adequately provide an economical and efficient process for converting cellulose pulp industry byproduct hemicelluloses, and other carbohydrates present in hemicaustic, into useful products.